1. Field of the Invention
The present invention relates to an ATM (Asynchronous Transfer Mode) network apparatus, and more particularly to a traffic shaping method and apparatus which controls the output rate when ATM cells are outputted to the outside of an ATM network apparatus.
2. Description of the Related Art
Conventionally, a traffic shaping apparatus of the type mentioned serves as an internal function of an ATM network apparatus and is generally constructed such that it outputs ATM cells whose output rate is controlled by a traffic shaper, before they come to an output of the ATM network apparatus, through a frame crossing-over circuit for crossing over the ATM cells to external operation frames.
In the conventional traffic shaping apparatus described above, a flow of traffic shaped ATM cells suffers from a fluctuation when it passes through the frame crossing-over circuit. This fluctuation sometimes generates a factor which gives rise to a violation to a peak rate.
FIG. 3 is a sequence chart of a signal illustrating an outline of operation from a conventional traffic shaping apparatus as described above. In ordinary operation, ATM cells are outputted regularly at a rate set in advance for the traffic shaper, However, if an overhead pointer 32 of a synchronous digital hierarchy (SDH) is inserted in the ATM cells, then outputting of an ATM cell is temporarily delayed by the inserted overheat pointer 32. Consequently, the interval between the ATM cell and the next ATM cell is reduced, producing a peak rate violate 33. When the peak rate violation 33 occurs, the outer party side of a UNI (User Network Interface) interprets it as a UPC (Usage Parameter Control) violation, and the cell is abandoned. Consequently, the conventional traffic shaping apparatus has the disadvantage in that regular communication is disturbed by occurrence of a peak rate violation caused by a fluctuation of the distance between ATM cells.
It is an object of the present invention to provide a traffic shaping method and apparatus which prevents such a fluctuation of the interval between ATM cells to be outputted causing a peak rate violation which disturbs regular communication.
In order to attain the object described above, the present invention provides a traffic shaping method; comprising a traffic shaping step of performing a traffic shaping operation while controlling a rate of ATM cells to be outputted from an ATM network apparatus, a crossing over step of crossing over a frame period of the ATM cells, whose output rate has been controlled by the traffic shaping step, from that of internal-operation frames to that of external operation frames, and a supervising step of supervising a transmission rate of the ATM cells outputted in the crossed over frame period.
Since the transmission rate of the ATM cells outputted in the crossed over frame period from the ATM network apparatus is supervised, the traffic shaping can be stopped based on a condition of the output rate, or when the average rate drops, the traffic shaping can be rendered effective until the average rate rises to a maximum rate in advance.
In particular, the supervising step may include a step of supervising the output rate of the ATM cells, and if an overhead including a pointer inserted in the ATM cells is detected, transmitting a back pressure which serves as a stopping signal to stop the traffic shaping operation.
Alternatively or additionally, the supervising step may include a step of supervising the output rate of the ATM cells and transmitting output rate information, and causing, when the average rate exhibits a drop, the traffic shaping operation to be performed until the average rate rises to a maximum rate set in advance.
According to another aspect of the present invention, there is provided a traffic shaping apparatus, comprising a traffic shaper for controlling a rate of ATM cells to be outputted from an ATM network apparatus, a frame crossing over circuit for crossing over a frame period of the ATM cells, whose output rate has been controlled by the traffic shaper, from that of internal operation frames to that of external operation frames, and an output rate supervision circuit for supervising a transmission rate of the ATM cells outputted in the frame period crossed over by the frame crossing over circuit.
The output rate supervision circuit may supervise the output rate of the ATM cells and, if an overhead including a pointer inserted in the ATM cells is detected, transmit a back pressure which serves as a stopping signal to the traffic shaper to stop the traffic shaping operation.
Alternatively or additionally, the output rate supervision circuit may supervise the output rate of the ATM cells and transmit output rate information to the traffic shaper, and cause, when the average rate exhibits a drop, the traffic shaping operation to be performed until the average rate rises to a maximum rate set in advance.
In the traffic shaping method and apparatus, since the transmission rate of ATM cells outputted in a crossed over frame period is supervised, if an overhead including a pointer inserted in the ATM cells is detected, then a back pressure can be transmitted to stop the traffic shaping operation. Consequently, useless retention of the cells can be prevented. Further, since output rate information can be transmitted so that, when an average rate of the ATM cells exhibits a drop, the traffic shaping operation can be stopped until the average rate rises to a maximum rate set in advance. The traffic shaping method and apparatus is advantageous also in that it can prevent such a situation that a flow of the ATM cells exhibits a fluctuation causing an apparatus which receives the flow of the ATM cells to suffer from a UPC violation and abandon a cell, resulting in obstruction to regular communication.